GoTo - File - Export - Export Data
Choose - Files of Type - Lithography (*.Stl) - Save
It would be best to check your file before upload.
Get a copy of http://www.netfabb.com/basic.php
You can check, repair and measure your model here.

There are a few other things you should know before you export.
On the command line in AutoCad. Type in these commands.
'facetres' Enter 10
'isolines' Enter 0
'dispsilh' Enter 1
'regen' Enter
This smooths out your model especially on curves, eliminates the shown mesh lines in wire-frame mode and the last command displays a silhouette line in wire frame-mode